People who watch BBC programmes only on iPlayer will be required to buy a TV licence to view the content from 1 September.
Previously a licence was only needed to watch live broadcasts, so catch-up content was technically exempt from the £145.50 annual fee.
But due to a change in the law, a licence will be needed to download or watch BBC programmes on demand.
Those who already have a TV licence will not be affected.
The change comes after the government said it wanted to modernise the current system, so those watching catch-up TV do not get "a free ride".
'All who watch pay for it'
"When the licence fee was invented, video on demand did not exist," former Culture Secretary John Whittingdale said in March.
"The BBC works on the basis that all who watch it pay for it. Giving a free ride to those who enjoy Sherlock or Bake Off an hour, a day or a week after they are broadcast was never intended and is wrong."
The new rules apply to all devices used to access iPlayer - including laptops, smartphones, tablets, TV streaming devices and games consoles, as well as through third-party services such as Sky, Virgin or BT.
However, a TV licence will still not be needed for watching other on demand services, such as ITV Player, All4, My5 or Netflix.
TV Licensing said fewer than 2% of households would be affected by the change and would "not affect the huge majority of households which are already licensed".
It added all unlicensed households would be notified of the change in law and a publicity campaign to promote awareness will be carried out before 1 September.
TV Licensing is also pushing awareness for students, many of whom will be affected at the start of the new academic year.